Process and compositions for the disinfection of waters

ABSTRACT

A process for killing microorganisms and controlling biofouling in high chlorine demand waters comprises mixing two components, one of which is an oxidant and the other an ammonium salt, and adding the mixture immediately to the aqueous system to be treated.

This application is a divisional of application Ser. No. 09/023,984,filed Feb. 13, 1998, now U.S. Pat. No. 6,478,973, which is acontinuation of application Ser. No. 08/639,708, filed Apr. 29, 1996,now abandoned, which is a continuation of application Ser. No.08/272,276, filed Jul. 8, 1994, now abandoned, which is a continuationof application Ser. No. 07/892,533, filed Jun. 1, 1992, now abandoned,which claims the benefit of Israeli Application No. 98352, filed Jun. 3,1991.

BACKGROUND OF THE INVENTION

Biological fouling of circulating water is a known and well documentedproblem. Several factors contribute to the problem and govern itsextent: water temperature; water pH; organic and inorganic nutrientseither from air drawn into the system or from materials naturallyoccurring in the water or continuously supplied during plant operation;aerobic/anaerobic conditions; the presence/absence of sunlight, etc.

Algae, fungi, bacteria, as well as other simple life forms are found incirculating water. The types of microorganisms and the extent ofmicrobial growth depend on the water source and on the other factors.

Biological growth in circulating water can foul pipelines, acceleratecorrosion, attack wood, decrease heat transfer, plug filters, causeimperfections in paper sheets; decompose sizing mixtures, and cause manyother process interferences.

Oxidizing biocides including chlorine gas, hypochlorous acid, bromineand other oxidizing biocides are widely used in recirculating water.

“Chlorine demand” is defined as the quantity of chlorine that is reducedor otherwise transformed to inert forms of chlorine by substances in thewater; and standard methods have been established for measuring it. Inthis specification and claims “chlorine demand” is as measured byprocedures outlined in “Standard Methods for the examination of waterand waste water,”, 16th edition, Methods §409, pages 316-319. Themethods are based on applying a specific dose of chlorine to the mediumand measuring the residual chlorine left after a given contact time.Chlorine-consuming substances include ammonia and amino derivatives;sulfides, cyanides, oxidizable cations, pulp lignins, starch, sugars,oil, water treatment additives like scale and corrosion inhibitors.

Microbial growth in the water and in biofilms contribute to the chlorinedemand of the water and to the chlorine demand of the system to betreated. Oxidizing biocides were found to be ineffective in waterscontaining a high chlorine demand, including heavy slimes. Non-oxidizingbiocides are usually recommended for such waters.

Chlorination of water having a high content of ammonia or otheramino-derivatives results in the formation of chloramines. Chloraminesare described as poor biocides relative to hypochlorous or hypobromousacid. According to literature, chloramines are slow to react and may bemore persistent in water systems (The NALCO water handbook, 1988, PCT/US89/02730 21.6.1989, Great Lakes Chem, Corp. Wat. Sci. Tech. 20 No 11/12,pp. 385-39, 1988, by M. D. Sobsey et al., National Academy of Science,1980, Drinking Water and Health, Vol. 2, National Academy Press,Washington, D.C.).

Chloramination of drinking water occurs when chlorine reacts with smallamounts of ammonia either present in or added to the water.

Traditional chloramination occurs with the addition of free chlorine tothe total amount of water for reaction with small amounts of ammoniapresent in the water, or added to the water in known amounts. Only onereference describes the use of pre-formed monochloramine for thepost-disinfection of drinking water (J. Beck et al., Aqua I, 25-33,1986). In this work, chloramines were formed by mixing ammonium sulfateand hypochlorite solution at a concentration of 1000 ppm; pH wasadjusted to 7.5 before the point of dosage to avoid carbonateprecipitation.

Chloramines were used to control aftergrowth and biofouling in thesurface seawater reverse osmosis plants (Desalination 74, 51-67 (1989)and European Patent Application No. 90108872.4, 11.05.90, for Du Pont deNemours and Company). This patent claims the use of chloramine toinhibit regrowth following dechlorination in liquid process streamscontaining chlorine degradable organic material, that when in degradedform provides energy and carbon source that is assimilable bymicroorganisms The chloramine for the process was made in situ by addingNH₃ gas, NH₄OH, NH₄Cl or (NH₄)₂SO₄. The sources of chlorine were Cl₂gas, NaOCl, Ca(OCl)₂ and electrolytically generated chlorine.

Chloramines formed in situ during chlorination of cooling watercontaining ammonia, is considered to have no biocidal effect in thetreatment of cooling towers, since chloramines are quickly stripped dueto their high volatility [G. Holz Warth et al., Water Res. 18(1),1421-1427 (1984].

The disinfection of highly turbid waste water using chlorine hadimproved when ammonia was added to the waste water (in situ), whenlonger contact times were allowed [Ataei Khalil Z. et al.; Proc. Annu.Conf. Am. Water Works Assoc., 1988 (Pt. 2), pp. 1763-1770].

Ammonium bromide was not mentioned as a possible source for chloramines.The usual sources are ammonia, ammonium chloride and ammonium sulfate.

Its is a purpose of the invention to provide a process and compositionsfor killing microorganisms and inhibiting biofouling in waters,especially in cooling waters and aqueous systems having a high chlorinedemand waters, and more especially in cooling waters and aqueous systemshaving a high chlorine demand.

It is another purpose of the invention to provide such a process andcompositions that have a high biocidal effect and a high initial rate ofkill in high chlorine demand waters.

It is further purpose of the invention to provide such a process andcompositions the biocidal effect and the properties of which areconstant and predetermined.

Other purposes and advantages of the invention will become apparent asthe description proceeds.

SUMMARY OF THE INVENTION

The process according to the invention comprises mixing two components,one of which is an oxidant, preferably an active chlorine donor andstill more preferably sodium hypochlorite, and the other, an ammoniumsalt, preferably chosen among halides, sulfates and nitrates, and addingthe biocidal concentrate immediately to the aqueous system to betreated. The frequency, duration and concentration should be determinedin each individual case so as to be sufficient to control biofouling.

Preferably, the two ingredients are mixed in a specific order, andspecifically the oxidant is added to a solution of the ammonium salt. Ina preferred form of the invention, the oxidant is NaOCl and is slowlyadded to a well-mixed solution of the ammonium salt diluted in the rangeof 0.01% to 2% equimolar to chlorine, preferably until a finalconcentration of chlorine in the mixture has reached 0.01-1% aschlorine. Either batch or continuous formation of the biocidal stocksolution is effective.

The biocidal mixture was found to be more effective than other oxidizingbiocides (such as chlorine or bromine) whenever the demand in the watersystem exceeds 1.8 ppm Cl₂ out of 2.0 ppm Cl₂ within 60 minutes. Thepercentage of ingredients are given as weight percent.

The mole ratio N/Cl is preferably 1:1. An excess of N may be used.

The temperature of the water to which the mixture is added, can be inthe region of 10-60° C. The temperature of the solution of ammonium saltshould be 10-30° C. when NaOCl is added. The pH is controlled by theconcentration of the NaOCl solution; preferably the pH range should be8.0-12.5. The active ingredient was effective at pH 7 and at pH 8. Somedecrease in efficacy was noted at pH 9.

The frequency and duration of treatment and concentrations of activeingredient needed in order to maintain good control of biofouling shouldbe determined in each individual case. However, good control wasachieved at a level of 3 mg/l as chlorine (4.2 kg NH₄Br for 1000 m³).

The biocidal mixture is very effective for shock treatment of fouledsystems, even in cases where the water demand is low, and enables theeffective daily use of oxdizing biocides. A level of 9 mg/l (aschlorine) is sufficient to clean a fouled system.

In preferred forms of the invention, the mixture is formed and fedeither batchwise or continuously by any suitable means, such as by aliquid metering pump or by gravity.

The invention comprises the solutions prepared as described above.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following are non-limitative examples of possible applications ofthe process:

Recirculating cool water

Brewery pasteurizer

Air washer

Evaporative cooling water

Scrubbers

Pond and lagoon water

Closed water-cooling systems

Food plant disinfection

Bleaching—pulp and paper

etc.

The process according to the invention is compatible with other watertreatment chemicals, corrosion and scale inhibitors, etc.

EXAMPLE 1 Efficacy in Buffer at pH-7,5 Against Pseudomonas sp.

Dosage: 1 ppm as Cl₂;

Chlorine demand: 0.1 ppm out of 1 ppm within 20 minutes

NH₄Br+NaOCl: stock concentration: 1000 ppm as Cl₂

Preparation of stock solution: NH₄Br was dissolved in de-ionized water(2761 ppm). NaOCl (2000 ppm gm Cl₂) was quickly added dropwise to theammonium bromide solution while stirring the mixture. The stock solutionwas used immediately.

TABLE I Survivors (cfu/ml) after time (minutes) Biocide 1 5 10 20 NaOCl4 × 10⁶ 1 × 10⁵  4 × 10² 0 NaOCl + NaBr 5 × 10³ 4 × 10² 4 × 10 0 (1:1)NH₄Br + NaOCl 6 × 10⁶ 2 × 10⁶ 5 × 10³ 0 (1:1) Control — — — 9 × 10⁶

Results in Table I indicate higher rates of kill for NaOBr and NaOCl ascompared to NH₄Br+NaOCl in water with low demand for chlorine.

EXAMPLE 2 Efficacy of Ammonium Bromide at Various pHs

Examined MO: bacillus mycoides

Dose: 2 ppm as Cl₂

NH₄Br+NaOCl: molar ratio 1:1; stock concentration: 0.5%;

NH₄Br+NaOCl was either pro-mixed or added in situ to the buffer.

Demand: 1.8 ppm out of 2 ppm of Cl₂ within 60 minutes.

TABLE II Survivors: cfu/ml pH-7.0 pH-8.0 pH-9.0 Treatment 60 m 180 m 24h 60 m 180 m 24 h 60 m 180 m 24 h NH₄Br + NaOCl 7 × 10⁴ 2 × 10⁴ 2 × 10²2 × 10³ 9 × 10⁴ 2 × 10² 1 × 10⁵ 9 × 10⁴ 1 × 10⁴ premix NH₄Br + NaOCl 2 ×10⁵ 6 × 10⁴ 2 × 10⁴ 1 × 10⁵ 7 × 10⁴ 1 × 10⁴ 2 × 10⁵ 1 × 10⁵ 1 × 10⁵ insitu NaOCl 1 × 10⁵ 1 × 10⁵ 1 × 10⁵ 3 × 10⁵ 1 × 10⁵ 1 × 10⁵ 1 × 10⁵ 1 ×10⁵ 1 × 10⁵ NaOCl + NaBr 1:1 2 × 10⁵ 1 × 10⁵ 1 × 10⁵ 3 × 10⁵ 2 × 10⁵ 2 ×10⁵ 2 × 10⁵ 1 × 10⁵ 1 × 10⁵ non-treated — 3 × 10⁶ 5 × 10⁶ 3 × 10⁶ — 1 ×10⁵ 5 × 10⁵ — 4 × 10⁵

Table II shows that pre-mixed (NH₄Br+NaOCl) a higher rate of kill ascompared to either NaOCl or NaOBr, as the demand for chlorine increases.Efficacy was slightly impaired at pH from 8.0 to 9.0.

EXAMPLE 3 Efficacy of NH₄Cl+NaOCl in Water Taken from a Citrus JuiceEvaporator; Comparison to Non-oxidizing Biocides

Water demand: higher than 30 ppm of Cl₂ (out of 30 ppm Cl₂) within 60minutes.

Concentration of stock solution (NH₄Cl+NaOCl): 1000 ppm

Algicol II is a quaternary ammonium salt.

TABLE III dose Survivors (cfu/ml) after time (hours) Biocide mg/l 1 24 7days Kathon 30 2 × 10⁶ 7 × 10² 7 × 10² MBT 30 1 × 10⁶ 1 × 10³ 8 × 10²Algicol II 100 5 × 10⁶ 4 × 10⁶ 9 × 10⁶ NH₄Cl + NaOCl 30 0 0 0 Control —6 × 10⁶ 3 × 10⁷ 1 × 10⁷

Results in Table III indicates that a mixture of NH₄Cl+NaOCl was moreeffective than 3 non-oxidizing biocides in water with high demand forchlorine.

EXAMPLE 4 Efficacy of Oxidizing and Non-oxidizing Biocides is a StarchSizing Mixture (Paper Industry)

Efficacy was measured at 60° C.

NH₄Br+NaOCl: stock concentration 0.1%.

Dosage: 30 ppm active ingredient.

Incubation temperature 60° C.

TABLE IV Surviving MOs (cfu/ml) after time (hours) Biocide 4 28 48 72MBT 9 × 10⁴ 4 × 10⁴ 2 × 10⁴ 2 × 10⁴ NaOCl + NaBr 4 × 10³ 6 × 10⁵ 3 × 10⁵3 × 10⁶ NaOCl 4 × 10³ 2 × 10⁶ 3 × 10⁶ 3 × 10⁶ NaOCl + NH₄Br 2 × 10  3 00 MIT 2 × 10⁴ 2 × 10³ 2 × 10³ 1 × 10³ DBNPA 3 × 10⁴ 2 × 10⁵ 2 × 10⁵ 1 ×10⁵ Control 1 × 10⁵ 3 × 10⁵ 8 × 10⁵ 7 × 10⁵

Results in Table IV prove that a mixture of NH₄Br+NaOCl is moreeffective than other oxidizing and non-oxidizing biocides in a highdemand medium.

EXAMPLE 5 Kinetics of Kill of Various Mixtures of Ammonium Salts Mixedwith NaOCl in Water from a Citrus Juice Evaporator

Dosage: 30 ppm as Cl₂

Demand: Higher than 30 ppm out of 30 ppm of Cl₂ during 10 minutes.

Stock concentration of NH₄X+NaOCl 0.1% as Cl₂

TABLE V residue as Cl₂ Surviving MOs after time free (total) after(minutes); total aerobic time (minutes) (total anaerobic) cfu/ml Biocide10 20 60 10 20 60 NH₄Cl + NaOCl 5(10) 1(2) 0(0) 5 × 10²(4 × 10²) 3 ×10²(2 × 10²) 1 × 10²(3) NH₄Br + NaOCl 0(0)  0(0) 0(0) 4 × 10²(4 × 10²) 5× 10²(4 × 10²) 5 × 10²(2 × 10²) NH₄NO₃ + NaOCl 6(9)  — 0(0) 4 × 10²(2 ×10²) 4 × 10²(7) 8 × 10(2) NaOCl 0(0)  0(0) 0(0) 2 × 10⁵(8 × 10⁴) 2 ×10⁷(3 × 10⁶) 1 × 10⁷(3 × 10) NaOCl + NaBr 0(0)  0(0) 0(0) 2 × 10⁷(4 ×10⁶) 1 × 10⁷(3 × 10⁶) 2 × 10⁷(2 × 10⁶) CONTROL — — — — — 2 × 10⁷(6 ×10⁶)

Results in Table V show that mixtures of ammonium salts and NaOCl areeffective in controlling aerobic and anaerobic microorganisms in waterwith high demand for chlorine. Control was achieved within 10 minutes.Under these conditions both NaOCl and NaOBr are impaired by the media.The mixture of NH₄Br+NaOCl did not leave a measurable residue after 10minutes, yet it was very effective in reducing viable counts within 10minutes.

EXAMPLE 6 Efficacy of Oxidizing Biocides in Water Taken from a PaperMill (Thick Stock of Pulp Slurry)

Dose: 15 mg/l Cl₂

Microorganisms were incubated at 37° C. (total aerobic viable counts).

Concentration of stock solution: 0.1% as Cl₂.

TABLE VI counts Viable cfu/ml after time (minutes) Biocide 30 70 25hours DBNPA 2 × 10³ 6 × 10  8 × 10  NaOCl 6 × 10³ 2 × 10³ 1 × 10⁵NaOCl + NaBr 1 × 10⁴ 5 × 10³ 2 × 10⁶ NH₄Br + NaOCl 1 0 0 CONTROL 7 × 10⁶— 1 × 10⁷

Results in Table VI prove higher efficacy for NH₄Br+NaOCl as compared toother oxidizing biocides in this heavily loaded water.

EXAMPLE 7 Efficacy of a Series of Biocides in Domestic Waste Containinga High Concentration of Amines

Contact time: 10 minutes

Dose: 60 ppm as Cl₂

Incubation temperature; 27° C.

Stock concentration: 0.2%

N as NH₃: 50 mg/l; pH: 6.10.

TABLE VII Residue Surviving MOs (cfu/ml) free (total) as Cl₂ after 10minutes Biocide after 10 minutes aerob. anaer. fungi NaOCl 0.2(1.5) 9 ×10² 3 × 10³ 2 × 10 NH₄Cl + NaOCl 3.0(9.0) 2 × 10² 4 × 10² 0 DBNPA   0(0)2 × 10⁶ 5 × 10⁵ 0 Control — 5 × 10⁷ 1 × 10⁶ 2 × 10

Results in Table VII prove that in the presence of a high NH₃concentration, NaOCl was less effective than pre-mixed NH₄Cl+NaOCl incontrolling microbial growth (in water with high demand for Cl₂); goodcontrol was measured after 10 minutes.

EXAMPLE 8 Efficacy of Oxidizing Biocides in Domestic Waste

Non-treated domestic waste roughly filtered; contact time: 10 minutes.

Stock concentration: 0.5% as Cl₂.

Dose: 20 ppm as Cl₂

TABLE VIII residue as Cl₂ ppm Fecal coli total count treatment free(total) cfu/ml cfu/ml NaOCl   0(0) 5 × 10⁵ 6 × 10⁶ NaOCl + NaBr (1:1)  0(0) 3 × 10⁵ 7 × 10⁶ NaOCl + (NH₄)₂SO₄ (1:1) 0.05(0.7) 3 × 10³ 3 × 10⁴non-disinfected 5 × 10⁵ 1 × 10⁷

Results in Table VIII prove that pre-mixing (NH₄)₂SO₄ with NaOClresulted in lower viable counts of both fecal coli and total count.

In waste water with high organic load, this disinfection method wassuperior to disinfecting with either NaOCl or NaOBr.

EXAMPLE 9 Efficacy of Biocide in the Presence of Anti-scale andCorrosion Inhibiting Treatment (CWC)

Stock concentration: 0.5% as Cl₂

Efficacy against pseudomonas sp.

CWC: 100 mg/l

pH: 9.0

Contact time: 5 hours

TABLE IX dose Survivors Survivors in treatment mg/l in buffer CWC (100mg/l) DBNPA 4 0 4 × 10⁶ BCDMH 4 0 3 × 10⁵ NH₄Br + NaOCl 1 0 1 × 10³ 5 02 × 10 

Results in Table IX prove that in the presence of scale and corrosioninhibitors, efficacy of various biocides was impaired to such an extentthat much higher dosages of biocides had to be fed in order to maintaingood control. The mixture of NH₄Br+NaOCl was less impaired by CWC andestablished good microbial and algeal control even in the presence ofCWC.

EXAMPLE 10 Pre-mixing Versus in situ Addition of Ammonium Salt and NaOCl

Water from a corn-processing plant; high demand for Cl₂.

Dose: 12 ppm

NH₄Cl+NaOCl: stock concentration: 1%

NH₄Br+NaOCl: stock concentration: 0.5%

Stock solutions were formed at pH 14.0; 7.0, 4.0 and in water.

For in situ addition both NH₄X and NaOCl were dissolved at theappropriate pH.

TABLE X Survivors cfu/ml (total count after time in minutes) bufferpH-14.0 pH-7.0 pH-4.0 H₂O Treatment 60 180 60 180 60 180 60 180 NH₄Br +NaOCl 1 × 10⁶ 8 × 10⁵ 2 × 10⁵ 1 × 10⁶ 1 × 10⁶ 1 × 10⁶ 6 × 10⁴ 1 × 10⁴NH₄Cl + NaOCl 1 × 10⁵ 1 × 10⁵ 6 × 10⁴ 6 × 10³ 7 × 10⁵ 1 × 10⁶ 5 × 10⁴ 7× 10³ NH₄Br + NaOCl 1 × 10⁶ 2 × 10⁶ — — 2 × 10⁶ 2 × 10⁶ 5 × 10⁶ — insitu NH₄Cl + NaOCl 8 × 10⁵ 2 × 10⁵ — — 1 × 10⁶ 1 × 10⁶ 2 × 10⁶ 2 × 10⁶in situ NaOCl — — — — — — 2 × 10⁶ 2 × 10⁶ NaOCl + NaBr — — — — — — 3 ×10⁶ 3 × 10⁶ Control 4 × 10⁶ —

Results in Table X prove that the efficacy exhibited by mixtures ofNH₄X+NaOCl depend on the pH and on the mode of formation of the stockmixture. In situ addition of the two ingredients to water resulted inlower efficacy at any of the examined pH's.

Stock mixture of NH₄Br+NaOCl was more effective when prepared in waterthan when prepared in buffer at pH 7.0. When the stock solution wasprepared at a high or at a low pH, it was less effective.

EXAMPLE 11 Dependence of Efficacy of Mixtures of NH₄Br+NaOCl on theConcentrations of Sock Solution

Work was carried out in industrial waste water.

Stock concentrates were prepared in buffer at pH 7.00.

Biocidal dose: 4 ppm as Cl₂.

TABLE XI Survivors cfu/ml treatment total count after time (minutes)NH₄Br + NaOCl 10 60 180   2% 6 × 10⁴ 1 × 10⁴ 2 × 10³   1% 2 × 10⁵ 3 ×10⁴ 3 × 10⁴  0.5% 7 × 10⁴ 2 × 10⁴ 4 × 10³ 0.01% 2 × 10⁶ — 2 × 10⁶ insitu 5 × 10⁵ 3 × 10⁵ 5 × 10⁵ NaOCl 9 × 10⁵ — 7 × 10⁵ Control 1 × 10⁶ — —

Results in Table XI prove that the efficacy exhibited by the mixturescorrelated with the concentration of stock solutions. The highestefficacy was measured with a stock concentrations equal to at 0.5% asCl₂. Similar trends were obtained when the stock solutions were preparedin water rather than in buffer (see Table X) (The high efficacy measuredin buffer at a level of 2% as Cl₂ results from the higher pH of thismixture.)

FIELD EXPERIMENTS EXAMPLE I Cooling Tower 1

-   -   Cooling tower; contained volume 1000 m³    -   Circulation rate 500 m³/h    -   Scale and corrosion inhibitor: CWC: 100 mg/l

The tower was controlled on low level (0.6-1.2 kg/day) of BCDMH feed.Use of BCDMH was effective as long as make-ups were softened inion-exchangers.

When CWC (100 mg/l of phosphonate) replaced the use of ion-exchangers,much higher dosages of BCDMH (4-5 kg/day) did not suffice to preventbiofouling and growth of algae.

The system was shock-fed with NH₄Br+NaOCl. Overall dosage: 75 litersNaOCl (10%) 12.6 kg NH₄Br The mixture was fed during 1.5 hours. Thisshock treatment cleaned the system.

A slug dose of 25 liters NaOCl (10% as Cl₂) (+4.2 kg NH₄Br) was then fedto the cooling tower once in two to three days. The cooling towerremained clean, with no apparent growth of biofilm or algae. Ameasurable residue of 0.6-0.4 ppm (as total chlorine) was measured inthe water 24 and 48 hours after feeding the mixture.

EXAMPLE II Cooling Tower 2

-   -   Corn processing plant.    -   Contained volume: 20 m³    -   Circulation rate: 300 m³/h    -   pH: 7.5-8.0    -   Water temperature: 36° C.-57° C.

This tower was treated with BCDMH (1.50-2.26 kg/day) daily. Due to avery high organic load in the water, growth of biofilm was very fast.Treatment with BCDMH was effective in controlling the daily grown films,but was not effective against heavy slimes which covered the coolingtower.

A daily feed of 3 liters NaOCl (7% as Cl₂), mixed with 0.35 kg NH₄Brcontrolled the daily newly formed biofilm as well as the slime and algaegrowth covering the cooling tower, and left a clean cooling system afterthree weeks of daily treatment avoiding the need for shock treatment.

EXAMPLE III Starch Sizing Mixture

Paper mill, starch sizing

Contained volume: 20 m³

Flow rate: 8.33 m³/h. (6% starch in H₂O),

pH about 8.0

Temperature: 50° C.-70° C.

Sizing mixture is recirculated in a size press through a filter (80microns). Circulation rate: 6 m³/h. The sizing mixture had beenpreviously treated with NaOCl (10% as Cl₂), which was fed every 8 hours(30 liters per portion). With this treatment, filters had to be washedonce every two hours.

Use of NaOCl was replaced by the use of a mixture of NH₄Br+NaOCl (stockconcentration 0.5% as Cl₂).

Feeding of NaOCl (13 liters of 10% as Cl₂) and NH₄Br (2.5 kg) threetimes a day (every eight hours) kept the filters in the size pressclean; the treatment with NH₄Br+NaOCl was compatible with a blue dyeadded to the sizing mixture, and did not bleach the blue starch, unlikeNaOCl.

A number of embodiments of the invention have been described forpurposes of illustration, but it will be understood that they are notlimitative and that the invention can be carried out by persons skilledin the art with many modifications, variations and adaptations, withoutdeparting from its spirit and from the scope of the appended claims.

1. A process for killing microorganisms and controlling biofouling inhigh chlorine demand water, which comprises mixing a solution of sodiumhypochlorite and a solution of an ammonium salt, in a molar ratiocalculated as N/Cl of at least 1:1, to form a biocidal mixture, theconcentration of said mixture being 0.01% to 1.0% as Cl₂, andimmediately adding said mixture to said water.
 2. A process according toclaim 1, wherein said water has a chlorine demand of at least 1.8 ppmout of 2.0 ppm Cl₂ after 60 minutes.
 3. A process according to claim 1,wherein said mixture is added to said water at a daily level of at least2 mg/l as Cl₂.
 4. A process according to claim 1, wherein thetemperature of said mixture is 10-30° C.
 5. A process according to claim1, wherein said mixture is fed continuously to the water to be treated.6. A process according to claim 1, wherein the concentration of saidsodium hypochlorite/ammonium salt mixture is from 0.1% to 0.5% Cl₂.
 7. Aprocess according to claim 1 wherein said sodium hypochlorite is addedto a solution of said ammonium salt.
 8. A process according to claim 7wherein said sodium hypochlorite is added to a well-mixed solution ofsaid ammonium salt in the range of 0.1% to 1%, until the fullconcentration of chlorine in said mixture has reached 0.1-1%.
 9. Aprocess according to claim 1, wherein said ammonium salt is ammoniumsulfate.
 10. A process according to claim 1, wherein said mixture has apH of 8.0-12.5.
 11. A process according to claim 1, wherein said wateris recirculating water.